Python Regular Expressions

Quick reference to Regular Expressions (and search) in Python.

Anchors
Function Does what
^Python Match "Python" at the start of a string or line.
Python$ Match "Python" at the end of a string or line.
\APython Match "Python" at the start of a string.
Python\Z Match "Python" at the end of a string.
\bPython\b Match "Python" at a word boundary.
\bfoo\B \B is nonword boundary: match "foo" in "food" and "fool" but not alone.
Python(?=!) Match "Python" if followed by an exclamation point.
Python(?!!) Match "Python" if not followed by an exclamation point.
Match settings
Setting Does what
<.*> Greedy repetition: matches "<python>perl>".
<.*?> Nongreedy repetition: matches "<python>" in "<python>perl>".
group(num=0) This method returns the entire match (or the specific subgroup num).
groups() This method returns all matching subgroups in a tuple (empty if no matches).
re.I Option flag: Performs case-insensitive matching.
re.L Option flag: Interprets words according to the current locale. This interpretation affects the alphabetic group (\w and \W), as well as word boundary behavior (\b and \B).
re.M Option flag: Makes $ match the end of a line (not just the end of the string) and makes ^ match the start of any line (not just the start of the string).
re.S Option flag: Makes a period (dot) match any character including a newline.
re.U Option flag: Interprets letters according to the Unicode character set. This flag affects the behavior of \w, \W, \b, \B.
re.X Option flag: Permits "nice" regular expression syntax. It ignores whitespace (except inside a set [] or when escaped by a backslash) and treats unescaped # as a comment marker.
Pattern syntax
Function Does what
^ Matches beginning of line.
$ Matches end of line.
. Matches any single character except newline. Using m option allows it to match newline as well.
[...] Matches any single character in brackets.
[^...] Matches any single character not in brackets.
re* Matches 0 or more occurrences of preceding expression.
re+ Matches 1 or more occurrence of preceding expression.
re? Matches 0 or 1 occurrence of preceding expression.
re{n} Matches exactly n number of occurrences of preceding expression.
re{n,} Matches n or more occurrences of preceding expression.
re{n,m} Matches at least n and at most m occurrences of preceding expression.
a|b Matches either a or b.
(re) Groups regular expressions and remembers matched text.
(?imx) Temporarily toggles on i, m, or x options within a regular expression. If in parentheses only that area is affected.
(?-imx) Temporarily toggles off i, m, or x options within a regular expression. If in parentheses only that area is affected.
(?: re) Groups regular expressions without remembering matched text.
(?imx: re) Temporarily toggles on i, m, or x options within parentheses.
(?-imx: re) Temporarily toggles off i, m, or x options within parentheses.
(?#...) Comment.
(?= re) Specifies position using a pattern. Doesn't have a range.
(?! re) Specifies position using pattern negation. Doesn't have a range.
(?> re) Matches independent pattern without backtracking.
\w Matches word characters.
\W Matches nonword characters.
\s Matches whitespace. Equivalent to [\t\n\r\f].
\S Matches nonwhitespace.
\d Matches digits. Equivalent to [0-9].
\D Matches nondigits.
\A Matches beginning of string.
\Z Matches end of string. If a newline exists it matches just before newline.
\z Matches end of string.
\G Matches point where last match finished.
\b Matches word boundaries when outside brackets. Matches backspace (0x08) when inside brackets.
\B Matches nonword boundaries.
\n, \t, etc. Matches newlines, carriage returns, tabs, etc.
\1 ... \9 Backreference: Matches nth grouped subexpression.
\10 Backreference: Matches nth grouped subexpression if it was matched. Otherwise it refers to the octal representation of a character code.
Special syntax
Syntax Does what
([Pp])ython&\1erl Backreference: Match python&perl or Python&Perl.
(['"])[^\1]*\1 Backreference: Single or double-quoted string. "\1" matches whatever the 1st group matched. "\2" matches whatever the 2nd group matched, etc.
R(?#comment) Matches "R". The rest is a comment.
R(?i)uby Case-insensitive while matching "uby".
R(?i:uby) Case-insensitive while matching "uby".
rub(?:y|le) Group only without creating the "\1" backreference.

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